1. Field of the Invention
The present invention concerns a method to determine a predetermined signal amplitude of an examination subject in a magnetic resonance measurement in which, given a pulse sequence, multiple RF pulses are radiated into an examination subject in a pulse series; the invention also concerns an MR system for this. The invention is particularly suitable to reduce spatial image inhomogeneities in MR exposures that are caused by the spatial variation of the RF field distribution in the examination subject.
2. Description of the Prior Art
In whole-body imaging, artifacts occur in the image in MR systems, particularly with systems employing high field strengths (for example 3 Tesla), which has previously prevented a wider prevalence of such examinations. These image artifacts are reinforced with the increase of the field strength B0 that is used. They occur increasingly at even higher field strengths and also increasingly affect the imaging of the head at these field strengths.
Artifacts and inconsistencies in MR imaging and spectroscopy due to inhomogeneous B1 fields (i.e. the radiated radio-frequency fields) have long been known in magnetic resonance engineering. With conventional methods it is not possible to directly affect the B1 homogeneity of RF fields, such that conventional methods are largely based on being made as insensitive as possible to the B1 inhomogeneity. For example, composite pulses and adiabatic pulses are used, but these pulses have a limited applicability with regard to achievable flip angles, phase response in the use for slice selection, pulse times and the absorption rate given the radiation of RF power into a body. For this reason, such pulses are typically used for the preparation of the magnetization but could not become accepted for use in the excitation and refocusing of the magnetization in imaging sequences.
Furthermore, imaging sequences are known that are inherently less sensitive to flip angle variations or magnetization preparations in order to reduce the sensitivity of a subsequent imaging sequence (see Madhuranthakam et al, “BI-insensitive fast spin echo using adiabatic square wave enabling of the echo train (SWEET) excitation”, Magn Reson Med 59 (6) 1386-1393, 2008). Furthermore, it is known to achieve a spatial modulation of the generated transversal magnetization via a simultaneous action of RF and gradient pulses on the spin system. The achievable homogeneity of two-dimensional or three-dimensional pulses is not limited in principle; however, these modulations lead to very long pulse times. These pulse times can be shortened with the possibility of the parallel emission with multiple RF channels (see Katscher et al, “Transmit SENSE”, Magn Reson Med 49 (1) 144-150, 2003 and Zhu, “Parallel excitation with an array of transmit coils”, Magn Reson Med 51 (4) 775-784, 2004). However, the achievable pulse times are always still too long, such that they cannot replace the previously common slice-selective or non-selective pulses in the prevalent imaging sequences. Methods for the compensation of B1 field inhomogeneities of a single excitation that use fewer partial trajectories are likewise known that scan only a few k-space points (see Saekho et al “Fast-kz three-dimensional tailored radiofrequency pulse for reduced B1 inhomogeneity”, Magn Reson Med 55, 719-724, 2006 and Setsompop et al “Parallel RF transmission with eight channels at 3 Tesla”, Magn Reson Med 56, 1163-1171, 2006.
A direct influence on the RF field is possible via the temporally simultaneous radiation of RF pulses with multiple spatially separated RF transmission coils or, respectively, RF channels. The generated RF field can be spatially modulated by adaptation of phase and amplitude values in multiple RF transmitters operated in parallel. The achievable homogeneity is essentially limited by the number of available parallel transmission channels. The method of parallel transmission has the advantage that it can be applied directly to all prevalent imaging methods without a temporal modification of the imaging sequences.
Periodic imaging sequences in which RF pulses with a predetermined flip angle and phase angle are radiated at specific temporal intervals are used almost exclusively in MR imaging. In modern fast imaging sequences, the RF pulses follow one another so quickly that the transverse and longitudinal magnetization generated by an RF pulse have not yet relaxed again before the following RF pulse. In other imaging sequences (such as multispin echo sequences or specific gradient echo sequences), the RF pulses also follow one another at short intervals in order to generate multiple different phase-coded MR signals. In this case, the evolution of the spin system is very complex and, under the circumstances, is already very difficult to calculate with the Bloch equations after only a few pulses. The extended phase graph algorithm (EPG) is a k-space-analog description of the Bloch equations for the evolution of the spins that are exposed to a series of hard pulses (see for example Hennig, “Echoes—how to generate, recognize, use or avoid them in MR-imaging sequences”, Conc Magn Reson 1991; 3:125-143 and Alsop, “The sensitivity of low flip angle RARE imaging”, Magn Reson Med 1997; 37:176-184). Here the spin system is described with the aid of different dephasing states, and the number of possible states grows three times faster than the number of RF pulses. Only one state is read out in an echo (thus the actual MR signal) depending on the sequence. The population of this state (i.e. the signal strength of the echo) is fed from many possible echo paths that are populated in the course of the RF series depending on the corresponding flip angles and phases of the applied pulses. The magnetization forming the echo can be unambiguously determined from the flip angles and phases of the pulses. The relaxation times must also be known given consideration of the relaxation.
The inverse problem—the calculation of flip angles and phases that leads to an echo train with predefined amplitudes—is not unambiguous. Also, no general methods are known that determine an indefinite solution for a complete echo train. However, there are methods known as “look ahead” methods that, starting from a magnetization state, calculate the required flip angle in order to come to a predefined signal amplitude with one pulse or a few pulses. It has been shown that flip angle-dependent equilibrium states can be prepared. With a continuous series of m flip angles with initial value α(m) and end value α(n+m), a switch can be made between the equilibrium state belonging to the initial or, respectively, end flip angles without generating strong signal fluctuations (see Alsop, “The sensitivity of low flip angle RARE imaging”, Magn Reson Med 1997; 37:176-184 and Hennig et al, “Multiecho sequences with variable refocusing flip angles; optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS)”, Magn Reson Med 2003; 49:527-535).
The possibility to generate a predefined magnetization with variable flip angles has previously been used in order to stabilize the signal amplitude in the echo train, for example, and to avoid signal fluctuations (see LeRoux et al, “Stabilization of echo amplitudes in FSE sequences”, Magn Reson Med 1993; 30:183-191). Furthermore, the possibility has been utilized to reduce the energy deposition in the body, i.e. the signal absorption or SAR (Signal Absorption Rate). Small flip angles are used in a segment of the echo train in which signals for outer k-space regions are acquired while the flip angle is gradually increased in order to generate the signals for the middle k-space regions (see the aforementioned articles by Busse et al and Hennig et al, and Hennig, “Calculation of flip angles for echo trains with predefined amplitudes with the extended phase graph (EPG)-algorithm: principles and applications to hyperecho and TRAPS sequences”, Magn Reson Med 2004; 51:68-80). This possibility is likewise used in order to slow the signal decay along the echo train and to enable longer echo trains for three-dimensional, fast spin echo acquisitions (see the aforementioned articles by Alsop and Hennig et al, as well as “Practical Implementation of Optimized Tissue-Specific Prescribed Signal Evolutions for Improved Turbo-Spin-Echo Imaging,” Mugler et al. III, Proc. Intl. Soc. Mag. Reson. Med., Vol. 11, (2003), pg 203 and “Three-Dimensional T2-Weighted Imaging of the Brain Using Very Long Spin-Echo Trains,” Mugler III et al., Proceedings 8th Annual Meeting of ISMRM (2000) pg 687).
Furthermore, it is possible to reduce the sensitivity of fast spin echo sequences to B1 field inhomogeneities. A magnetization state is hereby prepared on which—starting from subsequently generated echo amplitudes—optimally few of the employed flip angles of the applied pulses depend (see the aforementioned Madhuranthakam et al article).
However, the problem of reducing the artifacts due to B1 inhomogeneity (in particular at higher field strengths) in MR exposures continues to exist.